The present invention relates to the transmission of data over power lines, and particularly power lines which simultaneously serve as links for communications conducted according to the X-10 protocol.
A variety of systems which use power networks, for example in the home, as communication links have been proposed and developed. One group of systems, which are available, employs the X-10 protocol according to which a modulated carrier is transmitted in bursts synchronized to zero crossings of one or more mains voltage phases. According to the X-10 protocol, a binary "1" may be transmitted in the form a carrier burst at a zero crossing of the mains frequency Conversely, a binary "0" may be transmitted in the form of the absence of a carrier at a zero crossing. Information transmitted according to this protocol is detected by a receiver which senses the presence of the communications carrier during discrete intervals separated by half cycles of the mains frequency. X-10 receiving equipment monitors the power network during power voltage zero crossings and during intervals between such crossings to determine whether received carrier pulses are, in fact, X-10 transmissions. X-10 transmissions may employ, for example, a carrier frequency of the order of 120 KHz.
Another communications technique which has been proposed is known as the Electronic Industries Association PLBus power line carrier standard. As presently proposed, this standard uses an amplitude-shift-keyed carrier transmitted during successive active intervals which alternate with quiescent intervals, when no signal is present. The duration of each interval, active or quiescent, defines the value of a respective data bit. Although the interval rate according to this standard is higher than the X-10 pulse rate, there is a considerable likelihood that active and quiescent intervals of such a PLBus transmission will appear at points in time which cause this transmission to be erroneously identified by X-10 equipment as an X-10 transmission.